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UC3845B Schematic ( PDF Datasheet ) - American First Semiconductor
| Teilenummer | UC3845B |
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| Beschreibung | UC3842B/3B/4B/5B - High Performance Current Mode Controllers | |
| Hersteller | American First Semiconductor | |
| Logo |  |
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Gesamt 16 Seiten 
High Performance Current Mode Controllers
General Description
UC3842B/3B/4B/5B,UC2842B/3B
Package outline
The UC3842B/3B/4B/5B, series are high performance fixed
frequency current mode controllers. They are specifically designed for
Off–Line and dc–to–dc converter applications offering the designer a
cost–effective solution with minimal external components. These
integrated circuits feature a trimmed oscillator for precise duty cycle
control, a temperature compensated reference, high gain error
amplifier, current sensing comparator, and a high current totem pole
output ideally suited for driving a power MOSFET.
Also included are protective features consisting of input and
reference undervoltage lockouts each with hysteresis, cycle–by–cycle
current limiting, programmable output deadtime, and a latch for single
pulse metering.
These devices are available in an 8–pin dual–in–line and surface
mount (SO–8) plastic package as well as the 14–pin plastic surface
mount (SO–14). The SO–14 package has separate power and ground
pins for the totem pole output stage.
The UCX842B has UVLO thresholds of 16 V (on) and 10 V (off),
ideally suited for off–line converters. The UCX843B is tailored for
lower voltage applications having UVLO thresholds of 8.5 V (on) and
7.6 V (off).
Features
• Trimmed Oscillator for Precise Frequency Control
• Oscillator Frequency Guaranteed at 250 kHz
• Current Mode Operation to 500 kHz
• Automatic Feed Forward Compensation
• Latching PWM for Cycle–By–Cycle Current Limiting
• Internally Trimmed Reference with Undervoltage Lockout
• High Current Totem Pole Output
• Undervoltage Lockout with Hysteresis
• Low Startup and Operating Current
Vref
8(14)
RT/CT
4(7)
Voltage
Feedback
Input
2(3)
Output
Compensation
1(1)
VCC 7(12)
R
5.0V
Reference
VCC
Undervoltage
Lockout
R
Vref
Undervoltage
Lockout
Oscillator
+
-
Error
Amplifier
Latching
PWM
Gnd 5(9)
Pin numbers in parenthesis are for the D suffix SO-14 package.
Figure 1. Simplified Block Diagram
VC
7(11)
Output
6(10)
Power
Ground
5(8)
Current
Sense
3(5) Input
8
1
8
1
DIP-8
SOP-8
14
1
SOP-14
Pin Connections
Compensation 1
Voltage Feedback 2
Current Sense 3
RT/CT 4
8 Vref
7 VCC
6 Output
5 Gnd
(Top View)
Compensation 1
NC 2
Voltage Feedback 3
NC 4
Current Sense 5
NC 6
RT/CT 7
14 Vref
13 NC
12 VCC
11 VC
10 Output
9 Gnd
8 Power Ground
(Top View)
@ 2010 Copyright By American First Semiconductor
Page 1/16
 
UC3842B/3B/4B/5B,UC2842B/3B
VCC = 15 V
IO = 1.0 mA to 20 mA
TA = 25°C
VCC = 12 V to 25
TA = 25°C
2.0 ms/DIV
Figure 12. Reference Load Regulation
0
ÄÄÄ ÄÄÄÄÄÄÄÄÄVCC
Source Saturation
VCC = 15 V
(Load to Ground) 80 µs Pulsed Load
ÄÄÄÄÄÄÄÄÄÄÄÄÄ-1.0 TA=25°C
120 Hz Rate
ÄÄÄÄÄ-Ă2.0
ÄÄÄÄTA = -Ă55°C
3.0
ÄÄÄTA = -Ă55°C
ÄÄÄ ÄÄÄ2.0
TA = 25°C
ÄÄÄÄ ÄÄ1.0 Sink Saturation
(Load to VCC)
Gnd
ÄÄÄÄ ÄÄ0
0 200 400 600 800
IO, OUTPUT LOAD CURRENT (mA)
Figure 14. Output Saturation Voltage
versus Load Current
VCC = 30 V
CL = 15 pF
TA = 25°C
100 ns/DIV
Figure 16. Output Cross Conduction
2.0 ms/DIV
Figure 13. Reference Line Regulation
90%
VCC = 15 V
CL = 1.0 nF
TA = 25°C
10%
50 ns/DIV
Figure 15. Output Waveform
25
20
15
10
RT = 10 k
ÄÄÄÄCT = 3.3 nF
5 VFB = 0 V
ÄÄÄÄISense = 0 V
ÄÄÄÄTA = 25°C
0
0 10 20 30 40
VCC, SUPPLY VOLTAGE (V)
Figure 17. Supply Current versus Supply Voltage
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Page 6/16
6 Page 
UC3842B/3B/4B/5B,UC2842B/3B
From VO
Ri
Rd Cf
2.5V
2(3)
+
1.0mA 2R
EA
Rf
R
Rf ≥ 8.8 k
1(1)
5(9)
Error Amp compensation circuit for stabilizing any current mode topology except for boost and flyback
converters operating with continuous inductor current.
From VO
Rp Ri
2.5V
2(3)
+
1.0mA 2R
Cp Rd Cf
Rf
EA
R
1(1)
5(9)
Error Amp compensation circuit for stabilizing current mode boost and flyback
topologies operating with continuous inductor current.
Figure 32. Error Amplifier Compensation
MPS3904
8(14)
RT
From VO
RSlope
Ri
CT
Cf
Rd
4(7)
2(3)
Rf
1(1)
VCC
7(12)
R
Bias
R
5.0V Ref
+
-
+
-
Osc
+
1.0mA 2R
-m
S
Q
R
EA R 1.0V Comp/Latch
- 3.0m
m
5(9)
36V
7(11)
6(10)
5(8)
3(5)
The buffered oscillator ramp can be resistively summed with either the voltage
feedback or current sense inputs to provide slope compensation.
Figure 33. Slope Compensation
Vin
RS
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Page 12/16
12 Page | ||
| Seiten | Gesamt 16 Seiten | |
| PDF Download | [ UC3845B Schematic.PDF ] | |
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